1. Field of the Invention
The present invention relates to a method for manufacturing a substrate according to the precharacterizing portion of claim 1 and furthermore relates to a substrate according to the precharacterizing portion of claim 18 and to a method for manufacturing a stamper to be used in producing optical discs, comprising the exposing, developing and heating of a photosensitive film that has been coated onto a substrate.
2. Description of the Related Art
From JP-58-077044 there is known a method for enhancing the contact intensity of a photoresist film by producing a crosslinking reaction after applying a thin photoresist coating to a glass substrate or to a reflective film present on the substrate. Thus a reflective film is first formed on a glass substrate, after which a photoresist film is coated thereon in a thickness of about 100 Å. By increasing the temperature, a cross-linking reaction is produced between said two films, and a photoresist film having a thickness of about 1,000 Å is coated onto the thus cross-linked films, followed by exposure and development thereof. The said publication does not provide any information as regards the influence of the crosslinked undercoating on the photoresist film that is finally to be exposed and developed.
Optical discs contain information that is recorded in a geometric structure. More in particular, said geometric structure consists of pits and lands, which pits are arranged in concentrical circles or in a spiral in a surface of a respective optical disc. Also grooves are used as the structure of optical discs. When large numbers of such optical discs are to be produced, however, a stamper is placed in the mould of, an injection moulding machine, after which the optical discs are finally metallized and provided with a protective lacquer and a label or print.
The stamper manufacturing process as referred to in the introduction is known per se from the prior Dutch patent applications nos. 9400225 and 1007216 filed in the name of the present applicant, according to which process an unstructured stamper plate is prepared, which preparation comprises the cleaning died possible coating of an adhesion layer for the photoresist that is yet to be coated Subsequently, a negative photoresist is coated unto a thus prepared, unstructured stamper plate, which photoresist is selectively exposed to laser light, after which the selectively exposed photoresist film is heated and subsequently integrally exposed. Finally the integrally exposed photoresist is developed so as to provide the structured photoresist film, which is subsequently healed. In the direct stamper recording (DSR) process that is currently used, in respect of which the present applicant was granted Dutch patent no. 1009106 before, a stamper is formed directly on a nickel substrate by means of a galvanic process. The substrate that is used thereby actually consists of a circular nickel disc having a diameter of about 200 μm and a thickness of about 300 μm, onto which disc a novolak-based photoresist has been coated, using an adhesion layer. The function of said adhesion layer, which, on account of its thickness, can only be considered to be a “monolayer”, is to form a suitable surface onto which the novolak-based photoresist can be coated. In the subsequent step of the DSR process, the photoresist film that has been coated onto the substrate is selectively exposed by means of laser light, and subsequently heated and integrally exposed. Following the developing step, removal of the photoresist takes place, with the exception of the photoresist posts that remain behind on the nickel substrate. Said photoresist posts are subsequently hardened by using all exposure step with a wavelength in the deep UV region, followed by a heating step at a high temperature (hard bake) so as to strengthen the exposed, and thus crosslinked, regions additional by further crosslinking of polymer chains. A stamper obtained by using such a method thus comprises a nickel shell including photoresist posts. Further hardening of the photoresist posts makes it possible to produce at least 5000 replicas by means of such a stamper, which is used in the mould of an injection moulding machine for mass-producing CD replicas.
Research has shown that defects, such as stripes and stains, may be introduced on the surface of the nickel substrate during the galvanic process. In some cases said defects can be detected on an unprocessed nickel substrate by means of halogen light. After the above-described processing steps in the DSR process, however, such defects will show up. This phenomenon is assumed to be caused by a local difference in the diffraction pattern, which pattern is formed by the structure of photoresist posts. Not only the human eye is very sensitive to differences in the diffraction pattern, but also the electric signal from the stamper can be influenced by the quality of the substrates.
Additional research has moreover shown that standing waves occur in the photoresist film during the exposure step of the stamper manufacturing process. Said standing waves, which are caused by reflection of the substrate, result in the occurrence of alternating high and low exposure energy levels in the thickness of the resist, as a result of which photoresist posts of irregular shape and dimensions are eventually obtained, which is undesirable.